The present invention is related to a method for making a PLZT ceramic capacitor, and more particularly to including bismuth in the start materials, calcining at a low temperature and after sintering in a closed container, annealing to remove free lead oxide.
A silver doped PLZT (lead-lanthanum-zirconate-titanate) capacitor dielectric is described in the ancestral patent U.S. Pat. No. 4,027,209. The method disclosed there includes calcining the PLZT start materials at a peak temperature of 1232.degree. C., adding silver and glass, forming a capcitor body and sintering at from 1038.degree. to 1121.degree. C. Also in a subsequent progenitor patent U.S. Pat. No. 4,135,224, bismuth trioxide is added to the calcined PLZT material to reduce the required sintering temperature and to accelerate the incorporation of silver in the PLZT grains at sintering. In the latter patent, full densification could be achieved at a peak sintering temperature as low as 1150.degree. C. with no glass having been added. In the parent U.S. Pat. No. 4,219,866 issued Aug. 26, 1980, barium was added to the start materials to achieve a higher dielectric constant.
In the parent application, a ferroelectric dielectric was defined as being composed of a lead zirconate and a barium titanate wherein 0.07 and 0.16 molar parts of the lead are replaced by lanthanum, wherein from 0.10 to 0.40 molar parts of the zirconate are replaced by titanate, and wherein the atomic ratio of barium to lead is from 0.015 to 0.39. In the present disclosure, the term "antiferroelectric compound of lead-barium-lanthanum-zirconate-titanate" is to be interpreted as including only those compounds as defined and limited above. As in the parent patent, the dielectric compounds of this invention are preferably doped with silver and bismuth.
It is an object of the present invention to make such an antiferroelectric dielectric having a higher dielectric constant K without sacrifice in the stability of K over a broad operating temperature range.